Magnetron sputtering of rotating targets is well known in the art and is used extensively for producing a wide variety of thin films on a wide variety of substrates. A reasonable over-view of sputtering with rotating cathodes can be found in U.S. Pat. No. 5,096,562 (which is hereby incorporated by reference herein), as one example.
In the most basic form of rotating-target magnetron sputtering, the material to be sputtered is either formed in the shape of a tube or is adhered to the outer surface of a support tube made of rigid material. A magnetron assembly is disposed within the tube and supplies magnetic flux which permeates the target such that there is adequate magnetic flux at the outer surface of the target. The magnetic field is designed in a way such that it retains electrons emitted from the target so as to increase the probability that they will have ionizing collisions with the working gas, hence enhancing the efficiency of the sputtering process.
Fabrication cost for targets of some materials, in particular ceramic TCO materials, are relatively high in comparison to the cost of the raw materials. To improve the economy of these targets, it is desirable to increase the thickness of the target material. In this way, the target will have significantly more usable material while adding only minimally to the overall cost of the target. This is because the fabrication cost does not change significantly. The only significant increase is due to the additional raw material used. Thicker targets should have the added benefit of allowing longer production campaigns between target changes.
As mentioned, increasing the target thickness too much can result in inadequate magnetic flux at the target surface when using standard magnetron assemblies. The need for a magnetron design with higher magnetic flux is clear.
However, the effort to increase magnetic flux will usually create a new problem wherein the width of the turn-around is broadened. This, in turn, results in an increased relative erosion rate at the target ends and, therefore, a shortened target life due to target “burn-through”. This is contrary to the purpose of increasing the target thickness.